1. Field of the Invention
The present invention relates to an amplifier for converting a charge signal which converts a charge signal outputted from a charge generating sensor into a voltage.
2. Description of the Related Art
A charge generating sensor such as a piezoelectric element generates a charge in proportional to a load magnitude which is mechanically applied. Generally, the charge generating sensor is suitable for measuring a continuous dynamic pressure and the like and is used for a pressure sensor (in-cylinder pressure sensor) or the like for measuring a combustion pressure in a cylinder. A signal from the charge generating sensor is generally converted into a voltage signal by using an amplifier with an ultra-high input impedance, thus to extract the signal from the charge generating sensor. Referring to FIG. 9, the amplifier for converting a charge signal (so-called a charge amplifier) is mainly used and has a feedback capacitor C between input and output terminals of an amplifier A0 having an infinite gain with an opposite phase (antiphase).
However, the measurement using the charge generating sensor always has a problem, namely, the leakage of charge and, the leakage amount of charges is increased when the amplifier for converting the charge signal is connected. For example, upon measuring the pressure by connecting the amplifier for converting the charge signal to a pressure sensor using a piezoelectric element, the pressure is increased from a zero level and is returned thereto. The charge of the pressure sensor becomes negative by the leakage amount of the charges. Accordingly, the fluctuation in a zero point level of the signal outputted from the amplifier for converting the charge signal disturbs an accurate measurement thereof.
A description is given of the case of the fluctuation in the zero point level due to the leakage of charges when the charge amplifier is connected to the cylinder pressure sensor having the piezoelectric element attached to an engine combustion chamber and a combustion pressure in the cylinder is measured.
In a general engine having four-stroke cycle (intake→compression→combustion→exhaust), a piston reaches approximately a top dead center (TDC). Then, when an exhaust valve is closed and an intake valve is opened, the cylinder pressure becomes an atmospheric pressure in the case of a natural aspiration engine, and it becomes a pressure obtained by adding a boost pressure (e.g., 500 to 1,500 mmHg) to the atmospheric pressure in the case of a supercharged engine.
In this case, the piezoelectric effect of the sensor element generates the charges proportional to a cylinder pressure load. Assuming that the generated charge is designated by −q, the charge −q is charged to the feedback capacitor C of the charge amplifier. Further, the charge −q is converted into a voltage signal +V by the amplifier A0 and is outputted. Therefore, in the case where the signal level is at the zero level and the boost pressure is present when the cylinder pressure becomes the atmospheric pressure, the increasing level of the boost pressure, as a DC voltage component, to the zero level of the atmospheric pressure becomes the basic level of a combustion waveform which rises by a combustion pressure to be generated.
From the TDC to a bottom dead center (BDC) of the piston, the intake continues in the meantime and, the cylinder pressure is not highly changed and is maintained to approximately the basic level. Next, the piston reaches approximately the BDC, the intake valve is closed, and the compression starts from the BDC to the TDC. Simultaneously with the compression, the cylinder pressure starts to increase, the charge of the piezoelectric element is increased, and the charges are sequentially charged to the feedback capacitor C of the charge amplifier. Further, the voltage signal +V converted and outputted by the amplifier A0 is increased.
An ignition generates a combustion pressure just before the piston reaches the TDC (i.e. just before the maximum value of the compression pressure), and the generation of the combustion pressure rapidly increases the charges of the piezoelectric element. Further, the voltage signal +V converted and outputted by the amplifier A0 is also rapidly increased. Then, the signal outputted as the combustion pressure is at the above-mentioned basic level, that is, is the level of the atmospheric pressure in the case of the natural aspiration engine, while, it is a signal obtained by superimposing (or overlapping) to the DC voltage component of the boost pressure in the case of the supercharged engine.
After the cylinder pressure reaches the maximum level, the piston approaches the BDC from the TDC. Simultaneously therewith, the cylinder pressure changes to decrease, then, the polarity of the charges is inverted, and the charges start to feedback to the piezoelectric element. That is, upon viewing this phenomenon from the piezoelectric element side, the charge −q is charged in proportional to a stress when the positive stress (compression force) acts (or is applied) to the piezoelectric element by the combustion pressure. Further, the charge −q is inverted to the charge +q inversely proportional to the stress when the combustion pressure is decreased and the negative stress (tension) acts to the piezoelectric element. This phenomenon inverts the polarity of the feedback capacitor C and, thus, the polarity of the output signal is inverted.
After that, the piston reaches approximately the BDC and the exhaust valve is opened (the intake valve is still closed). When the combustion gas is exhausted while the piston approaches to the TDC, the cylinder pressure of the natural aspiration engine returns to the atmospheric pressure and the cylinder pressure of the supercharged engine returns to the boost pressure. One combustion cycle ends and the signal level returns to the level before starting the combustion cycle.
An ordinate of an oscillograph denotes the signal voltage and an abscissa denotes a time or a rotational angle of a crankshaft. Then, the oscillograph draws the change in cylinder pressure during one cycle, namely, a combustion waveform. In the case of the natural aspiration engine, the pressure rises from the atmospheric pressure (signal level) and the combustion ends and, then the signal level returns to the original one. In the case of the supercharged engine, the pressure rises from the level of the boost pressure (level of the DC voltage) and the rising combustion ends and, then, the signal level is returned to the level of the original boost pressure.
However, an actual electric circuit formed by connecting the sensor to the amplifier does not really have an infinite insulation resistance. Consequently, in the combustion cycle having rapid repetitions such as that of the engine combustion, the charges are leaked at each cycle and the leaked charges are converted into the negative signal-voltage. Referring to FIG. 10, a drift DV of the signal level generates in relative to an effective combustion pressure ECP, resulting the offset of the rising point of the waveform.
In the case of waveform data including the offset of the above-mentioned signal level, under circumstances using a high speed operating device with a large capacity such as the research and development stages, the combustion waveform for one cycle is extracted and processed from the continuous combustion waveforms. Further, the combustion waveform of the atmospheric pressure or the absolute pressure is assumed and the waveform can be analyzed. However, the application to commercial mass production vehicles has various problems to be solved.
That is, mounting a system for measuring the engine combustion pressure and the combustion waveform on the commercial vehicles needs the operation and process of many offset values by using an on board device for the drift of the generated signal level so as to obtain the correct signal level. Thus, an enormous processing capacity needs to be added to the on board device and the increase in size of the device and in costs is caused.
For example, Japanese Patent Publication No. 3123798 suggests means for solving the problems, by which a filter having a threshold value of 0.01 to 1.0 Hz is connected to an output terminal of the amplifier for converting the charge signal and low-frequency components of the pressure waveform are removed so as to substantially maintain the zero level. However, according to the technology suggested in the Publication, since a high pass filter removes the low frequency components of the combustion waveform, an AC coupling is substantially used and the whole DC components of the waveform are removed. Therefore, the accurate waveform analysis further needs the correction of removed DC components and the operating load necessary for the signal processing is not reduced.